The sun; SOHO's EIT (Extreme-Ultraviolet Imaging Telescope)
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The sun; SOHO's EIT (Extreme-Ultraviolet Imaging Telescope). Earth-moon system from Galileo-1990 (from NASA photo gallery). The astronomer Jean Jacques d’Ortous deMairan is usually credited with discovering “circadian rhythms” in heliotrope plants in 1729. Properties of Circadian Rhythms.

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The sun; SOHO's EIT (Extreme-Ultraviolet Imaging Telescope)

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The sun; SOHO's EIT (Extreme-Ultraviolet Imaging Telescope)

Earth-moon system from Galileo-1990

(from NASA photo gallery)

The astronomer Jean Jacques d’Ortous deMairan is

usually credited with discovering “circadian rhythms”

in heliotrope plants in 1729

Properties of Circadian Rhythms

  • Name is derived from “circa” (approximately) and “diem” (day)

  • When organisms are placed in constant conditions of DD or dim LL, they express a circadian rhythm with a period (t) of slightly less or slightly more than 24 hrs

  • These rhythms must be synchronized or “entrained” to local time so that the phase (f) of the internal rhythm corresponds to the f of the outside world.

Prokaryotes such

as cyanobacteria (in this case Synechococcus sp.) express

circadian rhythms in gene

expression with a t of about 24 hrs

(from Golden et al. 1999)

Fungi, such as the orange bread mold, Neurospora crassa, express

a wide variety of circadian patterns. In this case, patterns of

conidiation, or asexual spore formation, occur every 19-20 hrs.

(from Bell-Pedersen, 1999)

Complex, multicellular animals

express a wide variety of

circadian rhythms. In this case,

the activity/sleep and body

temperature patterns of a single

human subject is expressed

in actogram format. The average

t is 24.1-25 hrs

(from Aschoff, 1981)

What are Biological Clocks?

  • Simply put, “biological clocks” are mechanisms by which organisms can tell time. The enable living things to predict internal and external events.

    • Internal oscillator

    • Zeitgeber or synchronizer

    • Output pathways

  • These clocks predict periods of varying magnitude

    • Annual (circannual)

    • Daily (circadian)







Input Pathway

General Model of Circadian Clocks

The Hypothalamic Suprachiasmatic Nucleus (SCN) is the

Master Pacemaker for Nocturnal Rodents and Probably

Many, If Not All, Mammals

(from Bittman, 1984)

SCN 2.2 Cells Induce Circadian Rhythms in NIH3T3 Cells

(from Allen, Rappe,

Cassone and Eannest, 2000)

SCN2.2 Cells Restore Circadian Rhythms

NIH3T3 Cells Do Not





(from Earnest, Liang, Ratliff and Cassone, 1999)

Recent advances in molecular genetics has identified specific genes that are important for circadian rhythms in many different types of

organisms that are expressed rhythmically in clock-associated tissues. This is the expression pattern of mouse period genes in the SCN.

(from Maywood et al. 2000)

“Common” Elements of Circadian Clocks

Positive Elements

Negative Elements

Clock Genes

Output (CCGs)

Rhythmic Metabolism

And Behavior

Positive Elements in circadian loops:

kaiA in Synechococcus

wc1 and wc2 in Neurospora

Clk, cyc, clock and bmal1 in animals

Negative Elements in circadian loops:

kaiC in Synechococcus

frq in Neurospora

Per, cry and tim in animals

In Mammals, At Least, Pineal Rhythmicity Derives Primarily

From Rhythmic SCN Input via the Sympathetic Nervous System

(from Bittman 1984)

Pinealectomy has no effect of rodent locomotor rhythms in DD

(from Richter, 1967)

But Daily Administration

Of the Pineal Hormone

Melatonin Entrains Rat

Circadian Rhythms.

This Effect is Dose- and

Phase-Dependent with an

ED50 of 1-5 mg/kg (Cassone

et al. 1986; Warren et al. 1993),

Independent of the Pineal

Gland (Warren et al. 1993)

and Does Not Require

Injections (ie. Infusions

of Melatonin Work; Kirsch et


(from Redman, Ng, and Armstrong, 1983)

Circadian Rhythms in the Classroom

  • Visit Website for the NSF Center for Biological Timing to learn more about clocks


  • Also, there are specific classroom and home activities listed in that site at


A copy of this presentation can be found at the following website

Visit the Biological Clocks Program

At Texas A&M at

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